Gas sensor element

ABSTRACT

A sensor element includes an element base made of an oxygen-ion conductive solid electrolyte, an internal space provided inside the element base, an electrochemical pump cell that pumps oxygen in and out between the internal space and outside, and a porous thermal shock resistant layer provided to an outermost peripheral part in a predetermined range at one end part of the element base, at which a gas inlet is provided. A thermal diffusion time in a thickness direction of the thermal shock resistant layer is 0.4 sec to 1.0 sec inclusive. A thermal diffusion time at a leading end part of the thermal shock resistant layer covering the gas inlet at a farthest leading end position at the one end part is longest, and a thermal diffusion time at a pump surface is longer than a thermal diffusion time at a heater surface.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority from Japanese application JP2017-067315, filed on Mar. 30, 2017, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a sensor element provided to a gassensor configured to detect a predetermined gas component in measurementgas, and particularly relates to a configuration for preventingwater-induced cracking of the sensor element.

Description of the Background Art

A conventionally widely known gas sensor includes a sensor element madeof an oxygen-ion conductive solid electrolyte such as zirconia (ZrO₂)and provided with electrodes on a surface and inside thereof. Such a gassensor is used to determine concentration of a desired gas component inmeasurement gas. The sensor element potentially cracks due to thermalshock attributable to water droplets adhered on the surface of thesensor element. In some configurations, a protective layer (porousprotective layer) made of porous solid is provided to prevent suchwater-induced cracking.

In a publicly known gas sensor (refer to Japanese Patent ApplicationLaid-Open No. 2011-237222, for example) including a sensor elementprovided with such a porous protective layer, the porous protectivelayer is made of a single material such as silicon carbide or aluminumnitride and has a thermal conductivity and a specific surface areawithin predetermined ranges. This configuration reduces an amount ofwater infiltrating into the sensor element, thereby preventingwater-induced cracking of the sensor element.

In a publicly known manner (refer to Japanese Patent ApplicationLaid-Open No. 2016-29360, for example), ranges of values of a thermalconductivity λ and a product λρCp of the thermal conductivity λ, adensity ρ, and a specific heat Cp are defined to achieve such adesirable water repellency of the porous protective layer in a sensorelement that water droplets adhered to the porous protective layer ofthe sensor element are repelled due to the Leidenfrost phenomenon.

With the manner disclosed in Japanese Patent Application Laid-Open No.2016-29360, it is enabled to judge quality of the water resistance basedon quality of the water repellency. However, Japanese Patent ApplicationLaid-Open No. 2016-29360 does not disclose or suggest the quality of thewater resistance of a sensor element having an excellent waterrepellency.

How much water-induced cracking is likely to occur to a sensor elementdepends on the ease (speed) of heat transfer in the porous protectivelayer. However, the thermal conductivity, which is described in JapanesePatent Application Laid-Open Nos. 2011-237222 and 2016-29360, is aparameter indicating the likelihood of heat transfer, but is not aparameter indicating the ease of heat transfer.

The water-induced cracking attributable to adhesion of water dropletsmay occur locally at any portion of a sensor element, which contactswith measurement gas. However, Japanese Patent Application Laid-OpenNos. 2011-237222 and 2016-29360 each merely show one cross-section of asensor element (gas sensor element), and thus it is not necessarilyclear how the porous protective layer is included in a sensor element towhich the water-induced cracking is unlikely to occur.

SUMMARY

The present invention is directed to a sensor element provided to a gassensor configured to detect a predetermined gas component in measurementgas, and particularly relates to a configuration for preventingwater-induced cracking of the sensor element.

According to the present invention, a sensor element provided to a gassensor configured to detect a predetermined gas component in measurementgas includes: an elongated plate element base made of an oxygen-ionconductive solid electrolyte and having a gas inlet at one end part; atleast one internal space provided inside the element base andcommunicated with the gas inlet under predetermined diffusionresistance; at least one electrochemical pump cell including an outerpump electrode formed on an outer surface of the element base, an innerpump electrode provided facing the at least one internal space, and asolid electrolyte located between the outer pump electrode and the atleast one inner pump electrode, the at least one electrochemical pumpcell configured to pump oxygen in and out between the at least oneinternal space and outside; a heater buried in a predetermined range atthe one end part of the element base; and a porous thermal shockresistant layer provided to an outermost peripheral part in thepredetermined range at the one end part of the element base. Among twomain surfaces of the element base, a main surface closer to the gasinlet, the at least one internal space, and the at least oneelectrochemical pump cell than to the heater in a thickness direction ofthe element base is defined as a pump surface of the sensor element, anda main surface closer to the heater than to the gas inlet, the at leastone internal space, and the at least one electrochemical pump cell isdefined as a heater surface of the sensor element. In a formation rangeof the thermal shock resistant layer in a longitudinal direction of thesensor element, ranges obtained by equally dividing, in two, a rangeextending from a farthest leading end position at the one end part to anend part position of the heater on a side farther from the farthestleading end position are defined as a zone A and a zone B, the zone Abeing closer to the one end part. A range that is positioned on a sidefarther from the farthest leading end position than the zone B and inwhich the heater is not provided is defined as a zone C. A part thatcovers the gas inlet at the farthest leading end position at the one endpart is a leading end part of the thermal shock resistant layer. Theleading end part is not included in the zone A. The sensor element isconfigured and arranged such that: a thermal diffusion time in athickness direction of the thermal shock resistant layer is 0.4 sec to1.0 sec inclusive; and a relational expression below is satisfied ateach portion for the thickness direction of the thermal shock resistantlayer: thermal diffusion time at the leading end part>average value ofthermal diffusion times at the pump surface in the zone A, the zone B,and the zone C≥average value of thermal diffusion times at the heatersurface in the zone A, the zone B, and the zone C≥average value ofthermal diffusion times at each of two side surfaces in the zone A, thezone B, and the zone C.

The present invention excellently prevents water-induced cracking of agas sensor element, for example, when attached to an exhaust pipe of aninternal combustion such as an engine and used.

An object of the present invention is to provide a gas sensor element towhich water-induced cracking is unlikely to occur.

These and other objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view schematically illustrating astructure of a sensor element 100 along a longitudinal direction of thesensor element 100;

FIG. 2 is a schematic diagram of a section orthogonal to thelongitudinal direction of the sensor element 100, illustrating anarrangement relation between an element base 101, a surface protectivelayer 170, and a thermal shock resistant layer 180;

FIG. 3 is a graph plotting a threshold water wetting amount against athickness of the thermal shock resistant layer 180 based on a result ofa water resistance test;

FIG. 4 is a graph plotting the threshold water wetting amount against aporosity of the thermal shock resistant layer 180 based on the result ofthe water resistance test;

FIG. 5 is a diagram plotting a thermal diffusivity of a test pieceagainst a porosity thereof; and

FIG. 6 is a graph plotting a threshold water wetting amount against athermal diffusion time based on results shown in Table 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

<Outline of Gas Sensor>

FIG. 1 is a vertical cross-sectional view schematically illustrating astructure of a gas sensor element (hereinafter also simply referred toas sensor element) 100 according to the present preferred embodimentalong a longitudinal direction of the sensor element 100. The sensorelement 100 is a limiting-current type gas sensor element as a maincomponent of a gas sensor (not illustrated) configured to detect apredetermined gas component in measurement gas and measure theconcentration of the gas.

The sensor element 100 illustrated in FIG. 1 includes, as a mainstructure, an elongated plate element base 101 mainly made of ceramicscontaining zirconia (yttrium-stabilized zirconia), which is anoxygen-ion conductive solid electrolyte. Various kinds of components areprovided outside and inside the element base 101. The element base 101having such a configuration is dense and air-tight. The configuration ofthe sensor element 100 illustrated in FIG. 1 is merely exemplary anddoes not limit the specific configuration of the sensor element 100.

The sensor element 100 illustrated in FIG. 1 is a so-called serialdouble-chamber structure type gas sensor element including a firstinternal space 102 and a second internal space 103 which are internalspaces provided inside the element base 101. Specifically, in theelement base 101, the first internal space 102 is communicated, througha first diffusion control part 110 and a second diffusion control part120, with a gas inlet 104 externally opened at one end E1 of the elementbase 101 (to be precise, communicated through a thermal shock resistantlayer 180 to be described later), and the second internal space 103 iscommunicated with the first internal space 102 through a third diffusioncontrol part 130. A path from the gas inlet 104 to the second internalspace 103 is also referred to as a gas distribution part. In the sensorelement 100 according to the present preferred embodiment, thedistribution part is provided straight along a longitudinal direction ofthe element base 101.

The first diffusion control part 110, the second diffusion control part120, and the third diffusion control part 130 are each provided as twoslits on upper and lower sides in FIG. 1. The first diffusion controlpart 110, the second diffusion control part 120, and the third diffusioncontrol part 130 each apply predetermined diffusion resistance tomeasurement gas passing therethrough. A buffer space 115 is providedbetween the first diffusion control part 110 and the second diffusioncontrol part 120 to buffer pulsing of the measurement gas.

An external pump electrode 141 is provided on an outer surface of theelement base 101. An inner pump electrode 142 is provided in the firstinternal space 102. An auxiliary pump electrode 143 and a measurementelectrode 145 covered by a protective layer 144 are provided in thesecond internal space 103. A reference gas inlet 105 that is externallycommunicated and through which reference gas is introduced is providedat another end E2 of the element base 101. A reference electrode 147 isprovided in a porous alumina layer 146 communicated with the referencegas inlet 105 inside the element base 101.

For example, when NOx in measurement gas is measured by the sensorelement 100, the concentration of NOx gas in the measurement gas iscalculated through a process as described below.

First, having introduced into the first internal space 102, themeasurement gas is adjusted to have substantially constant oxygenconcentration by pumping operation of a main pump cell P1 (by pumpingoxygen in or out), and then introduced into the second internal space103. The main pump cell P1 is an electrochemical pump cell including theexternal pump electrode 141, the inner pump electrode 142, and a ceramiclayer 101 a as part of the element base 101 located between theelectrodes. Similarly, in the second internal space 103, oxygen in themeasurement gas is pumped out of the element by pumping operation of anauxiliary pump cell P2 as an electrochemical pump cell, so that themeasurement gas has sufficiently low oxygen partial pressure. Theauxiliary pump cell P2 includes the external pump electrode 141, theauxiliary pump electrode 143, and a ceramic layer 101 b as part of theelement base 101 located between the electrodes.

The external pump electrode 141, the inner pump electrode 142, and theauxiliary pump electrode 143 are formed as porous cermet electrodes (forexample, cermet electrodes made of Pt containing 1% of Au, and ZrO₂).The inner pump electrode 142 and the auxiliary pump electrode 143, whichcontact with the measurement gas, are each made of a material havingweakened or no reducing ability for an NOx component in the measurementgas.

NOx in the measurement gas made into the low oxygen partial pressurestate by the auxiliary pump cell is reduced or dissolved at themeasurement electrode 145 provided in the second internal space 103. Themeasurement electrode 145 is a porous cermet electrode also functioningas an NOx reduction catalyst that reduces NOx existing in an atmosphereinside the second internal space 103. The potential difference betweenthe measurement electrode 145 and the reference electrode 147 ismaintained constant through the reduction or dissolution. Oxygen ionsgenerated through the reduction or dissolution are pumped out of theelement by a measurement pump cell P3. The measurement pump cell P3includes the external pump electrode 141, the measurement electrode 145,and a ceramic layer 101 c as part of the element base 101 locatedbetween the electrodes. The measurement pump cell P3 is anelectrochemical pump cell configured to pump out oxygen generatedthrough the NOx dissolution in an atmosphere around the measurementelectrode 145. The sensor element 100 detects, in accordance with theamount of pumped out oxygen, pump current Ip2 flowing between themeasurement electrode 145 and the external pump electrode 141. The NOxsensor calculates the concentration of NOx in the measurement gas basedon the linear relation between the current value (NOx signal) of thepump current Ip2 and the concentration of dissolved NOx.

The pumping (pumping oxygen in or out) by the main pump cell P1, theauxiliary pump cell P2, and the measurement pump cell P3 is achievedwhen voltage necessary for pumping is applied between the electrodesprovided to each pump cell by a predetermined variable power source (notillustrated) as a component of the gas sensor, similar to the sensorelement 100. In a case of the measurement pump cell P3, voltage isapplied between the external pump electrode 141 and the measurementelectrode 145 so that the potential difference between the measurementelectrode 145 and the reference electrode 147 is maintained at apredetermined value. Typically, the variable power sources are providedfor each pump cell.

In the sensor element 100, a heater 150 is buried inside the elementbase 101. The heater 150 is provided, on the lower side of the gascirculation unit in FIG. 1, in a range extending from the vicinity ofthe one end E1 to the vicinities of positions at which the measurementelectrode 145 and the reference electrode 147 are formed. The heater 150is provided mainly to heat the sensor element 100 so that the oxygen-ionconductivity of the solid electrolyte is increased when the sensorelement 100 is used. For example, when used, the sensor element 100 isheated by the heater 150 so that the temperature reaches at 800° C. to850° C. approximately near the first internal space 102 heated to ahighest temperature. The heater 150 is a resistance heating element madeof, for example, platinum. More specifically, the heater 150 issurrounded by an insulating layer 151.

In the following description, among two main surfaces of the elementbase 101, a main surface (or an outer surface of the sensor element 100on which the main surface is provided) positioned on the upper side inFIG. 1, where the main pump cell P1, the auxiliary pump cell P2, and themeasurement pump cell P3 are mainly provided, is also referred to as apump surface, and a main surface (or an outer surface of the sensorelement 100 on which the main surface is provided) positioned on thelower side in FIG. 1, where the heater 150 is provided, is also referredto as a heater surface. In other words, the pump surface is a mainsurface on a side closer to the gas inlet 104, the two internal spaces,and the pump cells than to the heater 150, and the heater surface is amain surface on a side closer to the heater 150 than to the gas inlet104, the two internal spaces, and the pump cells.

A plurality of electrode terminals 160 are provided at the other end E2on the main surfaces of the element base 101 to achieve electricalconnection between the sensor element 100 and the outside. Specifically,in the sensor element 100 illustrated in FIG. 1, the four electrodeterminals 160 (160 a to 160 d) are provided on the pump surface, and thefour electrode terminals 160 (160 e to 160 h) are provided on the heatersurface. The electrode terminals 160 are electrically connected with theabove-described five electrodes, both ends of the heater 150, and aheater resistance detection lead (not illustrated) through lead wires(not illustrated) provided inside the element base 101, under apredetermined correspondence relations. With this configuration, voltageapplication to each pump cell in the sensor element 100 and heating ofthe heater 150 are performed through the electrode terminals 160.

In the sensor element 100, a part explained herein above, which includesthe element base 101 including the above-described first and secondinternal spaces and the other internal space such as the reference gasspace, and the various kinds of electrodes (including the protectivelayer 144) and the leads, the electrode terminals 160, the heater 150,and the insulating layer 151 provided to the element base 101, is alsoreferred to as a sensor element main part.

In addition, surface protective layers 170 (170 a and 170 b) areprovided on the pump surface and the heater surface of the element base101, respectively. The surface protective layers 170 are each made ofalumina, has a thickness of 5 μm to 30 μm approximately, and includespores at a porosity of approximately 20% to 40%. The surface protectivelayers 170 are provided to prevent adhesion of foreign objects andpoisoning materials onto the surfaces of the element base 101 and theexternal pump electrode 141 provided on the pump surface. Thus, thesurface protective layer 170 a on the pump surface functions as a pumpelectrode protective layer protecting the external pump electrode 141.

In the present preferred embodiment, the porosity is calculated byapplying a well-known image processing method (e.g. binarizationprocessing) to a scanning electron microscope (SEM) image of anevaluation target.

In FIG. 1, the surface protective layers 170 are provided substantiallyentirely across the pump surface and the heater surface except forexposed parts of the electrode terminals 160, which is, however, merelyexemplary. The surface protective layers 170 may be locally providednear the external pump electrode 141 at the one end E1 as compared tothe configuration illustrated in FIG. 1. Alternatively, no surfaceprotective layer 170 b may be provided on the heater surface.

In the sensor element 100, furthermore, the thermal shock resistantlayer 180, which is a porous layer made of alumina having a purity of99.0% or higher, is further provided at an outermost peripheral partwithin a predetermined range from the one end E1 of the element base101. FIG. 2 is a schematic diagram of a section orthogonal to thelongitudinal direction of the sensor element 100, illustrating anarrangement relation between the element base 101, the surfaceprotective layer 170, and the thermal shock resistant layer 180. FIG. 2omits illustrations of the electrodes and the internal spaces.

As understood from FIGS. 1 and 2, the thermal shock resistant layer 180entirely covers the one end E1 of the element base 101 and furthercovers, within a predetermined range from the one end E1 in the elementlongitudinal direction, not only the pump surface and the heater surfacebut also side surfaces of the element base 101. Thus, the thermal shockresistant layer 180 is disposed differently from the surface protectivelayer 170 provided only on the pump surface and the heater surface ofthe element base 101.

In the sensor element 100 having the above-described configuration, thesensor element main part and the surface protective layer 170, in otherwords, a part except for the thermal shock resistant layer 180 can bemanufactured through a well-known green sheet process. Specifically, aplurality of ceramic green sheets are subjected to predeterminedprocessing including punching for forming portions serving as internalspaces such as the first and second internal spaces after the completionof the sensor element, and printing of patterns of the electrodes andthe corresponding lead wires, the protective layer 144, the heater 150,the insulating layer 151, the surface protective layers 170, and thelike. Then, the plurality of ceramic green sheets are integrated witheach other by stacking and bonding to obtain a laminated body.Thereafter, element bodies obtained by dividing the laminated body intoindividual pieces are fired, so that the sensor element is obtained.Some sites such as the surface protective layers 170 may be formed byprinting corresponding patterns, not onto the green sheets, but onto thelaminated body yet to be divided into individual pieces.

On the other hand, the thermal shock resistant layer 180 is formed byapplying a well-known method such as plasma spraying, spray coating, gelcast, or dipping to a fired body (in other words, the sensor element 100except for the thermal shock resistant layer 180) obtained by formationof the sensor element main part and the surface protective layers 170through the green sheet process. Each method allows easy control of thethickness (film thickness) of the thermal shock resistant layer 180. Asfor plasma spraying, a resultant sprayed film includes pores due to acharacteristic of the method, and the porosity of the sprayed film canbe controlled by adjusting, for example, output power, an irradiationangle, and the property of powder material. As for gel cast or dipping,which uses alumina slurry as a row material, the porosity of the thermalshock resistant layer 180 can be controlled by controlling the conditionof a pore forming material added to the slurry. The tilt of an end faceof the thermal shock resistant layer 180 at the other end E2, which isexemplarily illustrated in FIG. 1, is likely to be provided when thethermal shock resistant layer 180 is formed by plasma spraying ordipping.

<Details of Thermal Shock Resistant Layer>

The thermal shock resistant layer 180 is provided to have a property ofwater resistance to mainly prevent so-called water-induced cracking ofthe sensor element 100. The water-induced cracking is a phenomenon whichoccurs to the sensor element 100, the element base 101 in particular(the element base 101 cracks), while the gas sensor is used, due tothermal shock attributable to water droplets adhered to the sensorelement 100 heated to high temperature by the heater 150. In some cases,the cracking of the element base 101 occurs with break of the electrodesand cracking of the surface protective layers 170.

Such water-induced cracking might occur, for example, in the case thatthe gas sensor including the sensor element 100 is disposed halfwaythrough an exhaust pipe of an internal combustion of an automobile orthe like, with the one end E1 of the sensor element 100 protruding intothe exhaust pipe. More specifically, when measurement is performed insuch a manner, the sensor element 100 is surrounded by a metalprotection cover through which the exhaust gas is allowed to be taken inand out, instead of being directly exposed in the exhaust pipe. Watervapor contained in the exhaust gas having entered into the protectioncover condenses and adheres to the sensor element 100 in some cases.Water-induced cracking occurs in such a case.

In the sensor element 100 according to the present preferred embodiment,the thermal shock resistant layer 180 is formed within a predeterminedrange from the one end E1, where water droplets are likely to adhere,and not over the entire sensor element 100, because it is provided tomainly prevent such water-induced cracking. Specifically, the thermalshock resistant layer 180 is formed within a range of 12 mm to 14 mmapproximately in the element longitudinal direction. The thermal shockresistant layer 180 may be formed within a range extending farthertoward the other end E2 in accordance with the configuration of thesensor element 100. In FIG. 1, the end face of the thermal shockresistant layer 180 at the other end E2 is tilted, but this is notessential.

The thermal shock resistant layer 180 is formed to have a thickness ofat least 200 μm or larger. If the thickness is smaller than 200 μm, thestrength of the thermal shock resistant layer 180 itself isinsufficient, and pores formed in the thermal shock resistant layer 180may penetrate through the thermal shock resistant layer 180 so thatwater vapor in the measurement gas is more likely to directly reach thesurface protective layer 170 or further at the element base 101. Thus,such a thickness is not preferable. As for the upper limit of thethickness, there is no particular restriction attributable to thefunctionality of the thermal shock resistant layer 180. However, if thethickness of the thermal shock resistant layer 180 is too large, themeasurement gas is unlikely to pass through the thermal shock resistantlayer 180 and reach the gas inlet 104, which degrades the responsivenessof the gas sensor and also leads to disadvantage in cost. Thus, such athickness is not preferable. For this reason, the thickness of thethermal shock resistant layer 180 is preferably 900 μm or smaller. Thethickness of the thermal shock resistant layer 180 can be evaluated bytransmissive X-ray irradiation.

The porosity of the thermal shock resistant layer 180 is preferably setto approximately 15% to 25% from the viewpoint of easiness anduniformity in manufacturing, and having less influence to intake of themeasurement gas into the element base 101 through the gas inlet 104.However, any value out of this range is applicable as long as theoccurrence of water-induced cracking is excellently prevented and theresponsiveness of the sensor element 100 is not affected.

<Method of Evaluating Water Resistance>

As described above, the thermal shock resistant layer 180 is a porouslayer provided to prevent the water-induced cracking of the sensorelement 100. Typically, the structure of a porous layer is defined basedon the thickness and porosity thereof, and thus, as a preliminary study,a water resistance test was first performed on 17 kinds of sensorelements 100 (No. 1 to 17) having different combinations of thicknessesand porosities of the thermal shock resistant layer 180. The thermalshock resistant layer 180 was formed by plasma spraying.

In the water resistance test, water droplets were dropped onto thethermal shock resistant layer 180 on the pump surface at a constant timeinterval equal to or shorter than 500 msec while each sensor element 100is heated by the heater 150 under a heating condition same as that foractual drive. The total amount of dropped water when cracking(water-induced cracking) occurred to the sensor element 100 was obtainedas a threshold water wetting amount, and the degree of the waterresistance was evaluated based on the magnitude of the threshold waterwetting amount. In other words, in the present preferred embodiment, thethreshold water wetting amount is used as an index value for theproperty of the water resistance. A larger threshold water wettingamount indicates that the property of the water resistance is moreexcellent.

In addition, in the water resistance test, images of the dropping ofwater droplets onto the thermal shock resistant layer 180 were capturedby a high-speed camera and played back to check whether water isrepelled due to the Leidenfrost phenomenon. The check confirmed thewater repellency for every sensor element 100. This indicates that theoccurrence of water-induced cracking in the water resistance test isattributable to thermal shock along with the dropping of water droplets.

Table 1 lists values of the threshold water wetting amount obtained byperforming a water resistance test on the total of 17 kinds of thesensor elements 100 with thicknesses and porosities of the thermal shockresistant layer 180, in ascending order of threshold water wettingamount. Each thickness was measured at the pump surface. FIG. 3 is agraph plotting a threshold water wetting amount against the thickness ofthe thermal shock resistant layer 180 based on a result of the waterresistance test. FIG. 4 is a graph plotting a threshold water wettingamount against the porosity of the thermal shock resistant layer 180based on the result of the water resistance test.

TABLE 1 Threshold water wetting Specimen No. Thickness (μm) Porosity (%)amount (μL) 1 449 16.3 3.0 2 443 15.7 3.0 3 430 18.4 3.5 4 438 15.3 3.55 456 14.6 3.5 6 445 16.0 4.0 7 427 18.5 4.5 8 457 18.1 4.5 9 413 22.05.0 10 418 21.7 5.4 11 419 19.0 5.4 12 445 18.4 6.4 13 408 23.9 6.9 14462 19.9 7.4 15 439 21.5 7.9 16 431 22.0 8.3 17 449 22.0 9.8

As shown in Table 1, the threshold water wetting amount was 3 μL orlarger for all of the 17 kinds of the sensor elements 100. Since theamount of condensed water in the exhaust pipe is 2 μL in the case thatthe gas sensor is attached to the exhaust pipe in the above-describedmanner, it can be said that each of the 17 kinds of the sensor elements100 has a sufficient property of water resistance.

However, FIG. 3 shows that there is no correlation between the thicknessof the thermal shock resistant layer 180 and the threshold water wettingamount at least in the thickness range (approximately 400 μm to 470 μm)shown in Table 1. In FIG. 4, an approximate straight line illustratedwith a solid line along data points has a positive gradient. Thus, thereis positive correlation between the porosity of the thermal shockresistant layer 180 and the threshold water wetting amount, but thedetermination coefficient R² as a square of a correlation coefficient Rwas only 0.60.

Accordingly, these results indicate that setting a thickness range or aporosity range alone does not immediately define requirements on thethermal shock resistant layer 180 having an excellent property of waterresistance.

Requirements on the property of the water resistance of the thermalshock resistant layer 180 having a favorable water repellency is assumedto depend on the ease of heat transfer in the thermal shock resistantlayer 180. Thus, if possible, it is more preferable to define the waterresistance based on an index directly representing the ease of heattransfer than using the thickness or the porosity as an index.

Generally, a thermal diffusivity is known as a parameter representingthe ease of heat transfer, which represents the ease of heat transferper constant area. When the ease of heat transfer is assumed to be samein a thickness direction and an in-plane direction orthogonal to thethickness direction for the thermal shock resistant layer 180, which isa porous layer including pores, the thermal diffusivity of the thermalshock resistant layer 180 is given by the following equation:

Thermal diffusivity=(Thickness)²/Thermal diffusion time  (1)

In the equation, the thermal diffusion time is a time required forthermal conduction in the thickness direction in the thermal shockresistant layer 180. The thermal conduction in the thickness directiontakes a longer time as the thermal diffusion time is longer.

The inventor made a study on evaluation of the water resistance by usingthe thermal diffusion time as an index. The reason is that thewater-induced cracking attributable to thermal shock is thought unlikelyto occur if the thermal diffusion time is sufficiently long for thethermal shock resistant layer 180, because heat applied to the outermostsurface of the thermal shock resistant layer 180 takes time to reach thesurface protective layer 170 or the element base 101.

Equation (1) can be rewritten as follows:

Thermal diffusion time=(Thickness)²/Thermal diffusivity  (2)

In other words, when the thicknesses and thermal diffusivity of thethermal shock resistant layer 180 is known, the thermal diffusion timein the thickness direction is specified.

However, although the thermal diffusivity can be typically measured fora bulk material by the well-known laser flash method, it is difficult toperform the measurement for the thermal shock resistant layer 180actually provided to the sensor element 100. The inventor of the presentinvention produced a plurality of test pieces (bulk specimens) havingporosities in an expected range of the porosity of the thermal shockresistant layer 180 by using an alumina material same as that of thethermal shock resistant layer 180 and measured the thermal diffusivityfor the test pieces by the laser flash method. Accordingly, it wasexperimentally confirmed that the thermal diffusivity and the porosityof each test piece have a relation (linear relation) therebetweenrepresented by a linear expression (α<0, β>0) below:

Thermal diffusivity=α·Porosity+β  (3)

FIG. 5 is a diagram plotting actually measured thermal diffusivities ofthe total of 12 kinds of test pieces against the correspondingporosities. The porosities of the produced test pieces wereapproximately 14% to 22%.

The expression of an approximate straight line illustrated in FIG. 5 isgiven by:

y=−0.0268x+0.7986  (4)

where x represents the porosity and y represents the thermaldiffusivity. The determination coefficient R² of the straight line has avalue of 0.8372.

Since Equation (3) (for example, Equation (4)) holds, the thermaldiffusivity of the thermal shock resistant layer 180 can be calculatedapproximately by obtaining the porosity of the thermal shock resistantlayer 180 actually provided to the sensor element 100 and thensubstituting the porosity into Equation (3) (for example, Equation (4)).Then, the thermal diffusion time of the thermal shock resistant layer180 in the thickness direction can be approximately obtained bysubstituting the thickness of the thermal shock resistant layer 180 andthe obtained thermal diffusivity thereof into Equation (2).

Table 2 lists, for the total of 17 kinds of the sensor elements 100whose thicknesses and porosities are shown in Table 1, thermal diffusiontimes in the thickness direction of the thermal shock resistant layer180 calculated based on Equations (4) and (2), together with thethreshold water wetting amounts shown in Table 1. FIG. 6 is a graphplotting the threshold water wetting amount against the thermaldiffusion time based on the results shown in Table 2.

TABLE 2 Threshold water wetting Thermal diffusion amount Specimen no.time (s) (μL) 1 0.51 3.0 2 0.48 3.0 3 0.53 3.5 4 0.46 3.5 5 0.48 3.5 60.49 4.0 7 0.53 4.5 8 0.59 4.5 9 0.66 5.0 10 0.65 5.4 11 0.53 5.4 120.57 6.4 13 0.77 6.9 14 0.69 7.4 15 0.71 7.9 16 0.71 8.3 17 0.78 9.8

In FIG. 6, an approximate straight line illustrated with a solid linealong data points has a positive gradient. In other words, there is apositive correlation between the thermal diffusion time in the thicknessdirection of the thermal shock resistant layer 180 and the thresholdwater wetting amount. The value of the determination coefficient R² is0.79, which is larger than the value of the determination coefficientR², 0.60, for the graph illustrated in FIG. 4 indicating the relationbetween the porosity of the thermal shock resistant layer 180 and thethreshold water wetting amount. This indicates that it is preferable touse, as an index, the thermal diffusion time in the thickness directioninstead of the porosity or the thickness when the requirements on thethermal shock resistant layer 180 are defined to secure a property ofwater resistance.

For this reason, in the present preferred embodiment, the thermaldiffusion time in the thickness direction of the thermal shock resistantlayer 180 is used as an index for evaluating the water resistance of thethermal shock resistant layer 180, and the requirements on the thermalshock resistant layer 180 are defined such that the thermal diffusiontime satisfies a predetermined range. The thickness and porosity of thethermal shock resistant layer 180 are defined to satisfy therequirements based on the thermal diffusion time. In this case, evenwhen the thermal shock resistant layer 180 is intentionally made to havedifferent thicknesses and porosities at different positions or hasunintentional non-uniformity and variance in the thickness and porositythereof, water-induced cracking is excellently prevented as long as thethermal diffusion time in the thickness direction satisfies apredetermined condition.

<Requirements on Thermal Shock Resistant Layer>

The following specifically describes requirements on the thermaldiffusion time in the thickness direction, which are to be satisfied bythe thermal shock resistant layer 180, based on the above description.

When the gas sensor is attached to an exhaust pipe, the amount ofcondensed water in the exhaust pipe is approximately 2 μL. According tothe graph illustrated in FIG. 6, the threshold water wetting amount isapproximately 2 μL or larger when the thermal diffusion time is 0.4 secor longer. Thus, a property of water resistance is secured at minimumwhen the thermal diffusion time of the thermal shock resistant layer 180in the thickness direction is 0.4 sec or longer.

Meanwhile, the upper limit value of the thermal diffusion time is 1.0sec from a viewpoint of the responsiveness of the sensor element 100.This is because, in view of Equations (2) and (3), increase of thethermal diffusion time thereof in the thickness direction requiresincrease of the thickness of the thermal shock resistant layer 180 orreduction of the porosity thereof, but either method hinders intake ofthe measurement gas though the gas inlet 104, and in particular, causessignificant decrease of the responsiveness in a range longer than 1.0sec.

Thus, in the sensor element 100 according to the present preferredembodiment, the thermal diffusion time in the thickness direction of thethermal shock resistant layer 180 is set to 0.4 sec to 1.0 sec. Withthis configuration, in the sensor element 100, the water-inducedcracking attributable to adhesion of water droplets is excellentlyprevented.

Accordingly, Table 2 and FIG. 6 show that the thermal shock resistantlayer 180 for which the thermal diffusion time in the thicknessdirection is approximately 0.45 sec to 0.8 sec has been actually formed.

However, although the water-induced cracking attributable to thermalshock due to adhesion of water droplets may occur locally at any portionof the sensor element 100, which contacts with the measurement gas, thesensor element main part has a non-uniform structure having a shape thatdiffers according to the position, and resistance against thermal shockdiffers according to the position. Thus, to more reliably prevent thewater-induced cracking, the thermal diffusion time is preferably longerat a site where the thermal shock resistance is relatively low.

Considering this point, in the present preferred embodiment, three zones(zone A, zone B, and zone C) are determined in the formation range ofthe thermal shock resistant layer 180 as illustrated in FIG. 1, and therequirements on the thermal shock resistant layer 180, which arenecessary for securing a property of water resistance, are defined withthese zones taken into consideration.

In the formation range of the thermal shock resistant layer 180 in thelongitudinal direction of the sensor element 100, the zones A and B areranges obtained by equally dividing, in two, a range extending from afarthest leading end position (the outer surface of the thermal shockresistant layer 180) at the one end E1 to an end part position of theheater 150 on a side farther from the farthest leading end position. Thezone A is the range closer to the one end E1, and the zone B is therange closer to the other end E2.

The zone C is a range positioned on a side farther from the farthestleading end position than the zone B in the formation range of thethermal shock resistant layer 180 in the longitudinal direction of thesensor element 100. In other words, the zone C is a range in which theheater 150 is not provided.

Although it depends on the formation ranges of the heater 150 and thethermal shock resistant layer 180, the ratio of the zones A, B, and C inthe longitudinal direction of the sensor element 100 is substantially1:1:1.

Specifically, the requirements on the thermal diffusion time in thethickness direction of the thermal shock resistant layer 180 are definedso that a first condition described below is satisfied. In the firstcondition, part of the zone A, which covers the gas inlet 104 at thefarthest leading end position at the one end E1, is referred to as a“leading end part” (of the thermal shock resistant layer 180) anddistinguished from the zone A.

(First condition): the thermal diffusion time of the “leading endpart”>the average value of the thermal diffusion times at the pumpsurface in the zones A to C≥the average value of the thermal diffusiontimes at the heater surface in the zones A to C≥the average value of thethermal diffusion times at each of two side surfaces in the zones A toC.

The first condition is based on consideration that the leading end partof the sensor element 100 disposed at a position closest to the centerof the exhaust pipe and provided with the gas inlet 104 has the lowestthermal shock resistance, and the pump surface side where a large numberof internal spaces are provided has the second lowest thermal shockresistance.

The thermal diffusion time at each surface in each zone may berepresented by the thermal diffusion time at a center part of thesurface in the zone.

Preferably, requirements on the thermal diffusion time in the thicknessdirection of the thermal shock resistant layer 180 is defined so that asecond condition is satisfied in addition to the first condition.

(Second condition): the thermal diffusion time at the pump surface inthe zones A and B>the thermal diffusion time at the pump surface in thezone C and the thermal diffusion time at the heater surface in the zonesA and B>the thermal diffusion time at the heater surface in the zone C.

The second condition is based on consideration that temperaturedifference from water droplets is more likely to occur in the zones Aand B, where the heater 150 is provided and which are heated to a highertemperature at operation of the gas sensor, and thus water-inducedcracking is more likely to occur.

As above, when the thermal shock resistant layer 180 is provided so thatthe first condition is satisfied or the first and second conditions aresatisfied, the sensor element 100 in which a property of waterresistance is sufficiently secured is achieved.

The thermal shock resistant layer 180 that satisfies the first conditionor satisfies the first and second conditions can be reliably formed byapplying the well-known method such as plasma spraying, spray coating,gel cast, or dipping described above and excellently adjustingconditions in each method. For example, the thermal shock resistantlayer 180 can be formed to have different thicknesses and porosities byapplying formation conditions different between sites.

As described above, according to the present preferred embodiment, thethermal shock resistant layer covering the element base, the thermaldiffusion time of which is 0.4 sec to 1.0 sec, is provided in the sensorelement at the outermost peripheral part in a predetermined range fromthe end part of the element base where the gas inlet is provided. Thisconfiguration excellently reduces the water-induced cracking of thesensor element, for example, when the sensor element is used while beingattached to an exhaust pipe of an internal combustion such as an engine.

In addition, the water-induced cracking can be more reliably preventedby increasing the thermal diffusion time at a site having a relativelylow thermal shock resistance.

<Modification>

The above preferred embodiment has described a case in which the thermalshock resistant layer 180 is provided to the serial double-chamberstructure type sensor element 100 having the gas inlet 104 at the oneend E1 of the element base. However, the configuration in which therequirements on the thermal shock resistant layer 180 are defined basedon the thermal diffusion time is not limited thereto. For example, theconfiguration is applicable to a serial triple-chamber structure typesensor element including three internal spaces.

Any sensor element that includes no internal space but to whichwater-induced cracking is likely to occur may include a thermal shockresistant layer for which the thermal diffusion time is adjusted so thatno water-induced cracking occurs.

Example

A thermal shock resistant layer, the thermal diffusion time of whichranges from 0.4 sec to 1.0 sec and satisfies the first condition, wasformed by plasma spraying.

Specifically, a fired body that had been prepared in advance and onwhich the sensor element main part and the surface protective layer 170had been formed was disposed at a predetermined position in a plasmaspraying device. Then, plasma spraying was performed while the firedbody was rotated and a position at which plasma from a plasma gun isincident on the sensor element main part was adjusted to shape a thermalshock resistant layer. Accordingly, the thermal shock resistant layerwas formed.

The thickness and porosity of the thermal shock resistant layer thusobtained were evaluated, and the thermal diffusion time in the thicknessdirection thereof was calculated at various positions by using Equations(2) and (4).

Table 3 lists, with thicknesses and porosities, thermal diffusion timesat various positions on the obtained thermal shock resistant layer andaverage values thereof in common parts (the leading end, the pumpsurface, the heater surface, and two side surfaces). A side surface 1and a side surface 2 in Table 3 correspond to parts of the thermal shockresistant layer that are formed along two facing side surface parts ofthe element base 101. When the sensor element 100 is disposed in theposture illustrated in FIG. 1, the side surface 1 is positioned on thefarther side in FIG. 1, and the side surface 2 is positioned on thecloser side in FIG. 1. The leading end (center) corresponds to a centralpart of the “leading end part” of the sensor element 100 in a front viewof the “leading end part”, and the leading end (edge) corresponds to apart of the “leading end part”, which is positioned above a thermalshock resistant layer formed along the side surface 2 of the elementbase 101 in the front view of the “leading end part”.

TABLE 3 Thermal Thickness Porosity diffusion Average Zone Position (μm)(%) time (s) value (A) Leading end (center) 388 25.9 0.90 0.94 (A)Leading end (edge) 417 25.4 0.97 A Pump surface 325 25.0 0.56 0.58 BPump surface 330 25.3 0.60 C Pump surface 317 25.8 0.59 A Heater surface322 23.9 0.48 0.47 B Heater surface 317 24.3 0.49 C Heater surface 29724.7 0.45 A Side surface 1 296 24.5 0.44 0.42 B Side surface 1 288 24.80.43 C Side surface 1 271 25.3 0.40 A Side surface 2 307 24.9 0.49 0.46B Side surface 2 301 25.0 0.48 C Side surface 2 281 24.9 0.41

Table 3 indicates that the thermal diffusion time at each positionranges from 0.4 sec to 1.0 sec and satisfies the first condition.

This result shows that it is actually possible to form a thermal shockresistant layer, the thermal diffusion time in the thickness directionof which ranges from 0.4 sec to 1.0 sec and satisfies the firstcondition.

While the invention has been shown and described in detail, theforegoing description is in all aspects illustrative and notrestrictive. It is therefore understood that numerous modifications andvariations can be devised without departing from the scope of theinvention.

What is claimed is:
 1. A sensor element provided to a gas sensorconfigured to detect a predetermined gas component in measurement gas,the sensor element comprising: an elongated plate element base made ofan oxygen-ion conductive solid electrolyte and having a gas inlet at oneend part; at least one internal space provided inside said element baseand communicated with said gas inlet under predetermined diffusionresistance; at least one electrochemical pump cell including an outerpump electrode formed on an outer surface of said element base, an innerpump electrode provided facing said at least one internal space, and asolid electrolyte located between said outer pump electrode and said atleast one inner pump electrode, the at least one electrochemical pumpcell configured to pump oxygen in and out between said at least oneinternal space and outside; a heater buried in a predetermined range atsaid one end part of said element base; and a porous thermal shockresistant layer provided to an outermost peripheral part in thepredetermined range at said one end part of said element base, whereinamong two main surfaces of said element base, a main surface closer tosaid gas inlet, said at least one internal space, and said at least oneelectrochemical pump cell than to said heater in a thickness directionof said element base is defined as a pump surface of said sensorelement, and a main surface closer to said heater than to said gasinlet, said at least one internal space, and said at least oneelectrochemical pump cell is defined as a heater surface of said sensorelement, in a formation range of said thermal shock resistant layer in alongitudinal direction of said sensor element, ranges obtained byequally dividing, in two, a range extending from a farthest leading endposition at said one end part to an end part position of said heater ona side farther from the farthest leading end position are defined as azone A and a zone B, the zone A being closer to said one end part, arange that is positioned on a side farther from said farthest leadingend position than said zone B and in which said heater is not providedis defined as a zone C, a part that covers said gas inlet at saidfarthest leading end position at said one end part is a leading end partof said thermal shock resistant layer, the leading end part is notincluded in said zone A, and said sensor element is configured andarranged such that: a thermal diffusion time in a thickness direction ofsaid thermal shock resistant layer is 0.4 sec to 1.0 sec inclusive, anda relational expression below is satisfied at each portion for thethickness direction of said thermal shock resistant layer: thermaldiffusion time at said leading end part>average value of thermaldiffusion times at said pump surface in said zone A, said zone B, andsaid zone C≥average value of thermal diffusion times at said heatersurface in said zone A, said zone B, and said zone C≥average value ofthermal diffusion times at each of two side surfaces in said zone A,said zone B, and said zone C.
 2. The gas sensor element according toclaim 1, wherein a relational expression below is further satisfied ateach portion for the thickness direction of said thermal shock resistantlayer: thermal diffusion time at said pump surface in said zone A andsaid zone B>thermal diffusion time at said pump surface in said zone C,and thermal diffusion time at said heater surface in said zone A andsaid zone B>thermal diffusion time at said heater surface in said zoneC.
 3. The gas sensor element according to claim 1, wherein said thermalshock resistant layer has a thickness of 200 μm to 900 μm inclusive. 4.The gas sensor element according to claim 1, further comprising asurface protective layer formed on at least part of said pump surface ofsaid element base or on at least part of said pump surface and saidheater surface, wherein said thermal shock resistant layer is in contactwith said element base and said surface protective layer.
 5. The gassensor element according to claim 2, wherein said thermal shockresistant layer has a thickness of 200 μm to 900 μm inclusive.
 6. Thegas sensor element according to claim 2, further comprising a surfaceprotective layer formed on at least part of said pump surface of saidelement base or on at least part of said pump surface and said heatersurface, wherein said thermal shock resistant layer is in contact withsaid element base and said surface protective layer.
 7. The gas sensorelement according to claim 3, further comprising a surface protectivelayer formed on at least part of said pump surface of said element baseor on at least part of said pump surface and said heater surface,wherein said thermal shock resistant layer is in contact with saidelement base and said surface protective layer.
 8. The gas sensorelement according to claim 5, further comprising a surface protectivelayer formed on at least part of said pump surface of said element baseor on at least part of said pump surface and said heater surface,wherein said thermal shock resistant layer is in contact with saidelement base and said surface protective layer.